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BASIC RADIO THEORY
1. Introduction
2. Basic principles
Phase difference
Frequencies
Radio signal classification
3. Modulation
AM
FM
Pulse
4. Antennas
Electromagnetic wave Characteristics
Types of antennas
Polarisation
5. Wave propagation
Factors
Surface propagation
Sky propagation
Space propagation
Doppler effect
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1- INTRODUCTION
The wave travels at a constant speed.
The form of the wave moves outward.
The wave becomes smaller as it moves awayfrom the source.
The wave is sinusoidal.
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2- BASIC PRINCIPLES
Cycle: a complete sequence of positive and
negative values (passing twice through one point).
Wavelength (): the physical distance traveled by a
radio wave during one cycle of transmission.
Amplitude (A): the maximum deflection/height of the
wave. It can be positive or negative.
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Velocity (c): the speed and direction of movement
through a given medium. Is the speed of the light(300 000 000 m/sec. = 162 000 NM/sec.)
c = 300 106 m/s
c = f = cte
Period (T): the duration of one cycle. T = 1 / f
Phase (): the fraction of one wavelength expressed
in degrees.
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Frequency (f): the number of cycles
occurring in one second in a radio wave.Expressed in Hertz (Hz).
1hz = 1 cycle/sec.
The higher the frequency, the shorter the
wavelength, and vice versa.
Kilo: Khz = 103 hzMega: Mhz = 103 Khz = 106 hz
Giga: Ghz = 103 Mhz = 106 Khz = 109 hz
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2.2- FREQUENCIES
Very Low Frequency (VLF) 3 30 Khz
Low Frequency (LF) 30 300 Khz Medium Frequency (MF) 300 3000 Khz
High Frequency (HF) 3 30 Mhz
Very High Frequency (VHF) 30 300 Mhz
Ultra High Frequency (UHF) 300 3000 Mhz Super High Frequency (SHF) 3 30 Ghz
Extremely High Frequency (EHF) 30 300 Ghz
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2.3- RADIO SIGNAL CLASSIFICATION
A radio signal may be classified by three
symbols in accordance with the ITU radioregulation.
First symbol indicates the type of modulation of
the main carrier.
Second symbol indicates the nature of the signalmodulating the main carrier.
Third symbol indicates the nature of the
information to be transmitted.
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3- MODULATION
Modulation: is the technical term for the
process of impressing and transportinginformation by radio waves.
Carrier wave: is the radio wave acting as the
carrier or transporter.
Keying: Interrupting the carrier wave to breakit into dots and dashes. Morse Code.
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3.1- AMPLITUDE MODULATION
AM: The information is impressed onto the
carrier wave by altering the amplitude of thecarrier.
Used in ADF
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3.2- FREQUENCY MODULATION
FM: The information is impressed onto the
carrier wave by altering the frequency of thecarrier.
Used in VOR, Radio Altimeters, Doppler.
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3.3- PULSE MODULATION
PULSE MODULATION: A modulation form used in
radar, by transmitting short pulses followed by largerinterruptions.
Phase Modulation: used in GPS where the phase of
the carrier wave is reversed.
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4.- ANTENNAS
A wave type transducerfor the process of
converting a line AC into afree
electromagnetic wave.
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4.2- ANTENNAS CHARACTERISTICS
The simplest type of antenna is a dipole which is a
wire of length equal to one half of the wavelength.**
In a wire which is fed with an AC (alternating
current), some of the power will radiate into space.
In a wire parallel to the wire fed with an AC butremote from it, an AC will be induced.
Aerial feeders are the connection between the
transmitter/receiver and the aerial. The type of
feeder depends upon the frequency to be used.
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4.3- TYPES OF ANTENNAS
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The common different kinds of directional
antennas are:
Loop antenna used in old ADF receivers.
Parabolic antenna used in weather radars.
Slotted planar array used in more modernweather radars.
Helical antenna used in GPS transmitters.
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4.4- POLARISATION
The polarisation of an electromagnetic wavedescribes the orientation of the plane of
oscillation of the electrical component of the
wave with regard to its direction of
propagation.
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A vertically polarised antenna transmits an
electromagnetic wave with theE field perpendicular
to the Earth's surface. Simple vertical antennas can
be used to provide Omnidirectional / all directions
communication.
A Horizontally polarised antenna transmits a radio
wave with the E field parallel to the Earth's surface.
A simple horizontal antenna is bi-directional. This
characteristic is useful when you desire to minimize
interference from certain directions.
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5- WAVE PROPAGATION
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5.1- FACTORS ON PROPAGATION
Reflection: Some surfaces exhibit reflective
properties to radio energy, accepting theincoming signal and then re-radiating it.
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Refraction: The speed of the radio wave is
affected differently on either side of thecentreline of the ray. Bends the wave.
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Diffraction: A radio wave travels close to the
Earths surface. The signal is attenuated andslowed. Bends and follows the Earths
curvature.
Attenuation of an RF signal is the reductionin signal strength due to absorption,
scattering or dispersion and diffraction.
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5.2- SURFACE PROPAGATION
The surface waves travel along the surface
of the Earth, attenuates quite quickly owingto absorption.
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5.3- SKY PROPAGATION
The sky waves are refracted by the ionosphere and
returned to Earth. It allows the signal field to carry
over much larger distances than the surface wave.
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Ionosphere is the ionized component of the
Earths upper atmosphere. It is from 60 to 400 km above the surface.
It is vertically structured in three regions or
layers (layers D, E and F) and their depth
varies with time.
The electromagnetic waves refracted from
the E and F layers of the ionosphere are the
sky waves.
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5.4- SPACE PROPAGATION
The space waves are line of sight, they
travel through the air directly from thetransmitter to the receiver.
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5.5- DOPPLER EFFECT
The Doppler effect is the phenomena that the
frequency of an electromagnetic wave willincrease or decrease if there is relativemotion between the transmitter and thereceiver.
The frequency will increase if the transmitterand receiver are converging.
The frequency will decrease if they arediverging.